Automated catheter length determination for implantable fluid delivery device

ABSTRACT

Disclosed are methods and systems for automatically estimating the length of an implantable catheter by measuring the pressure decay response to pumping fluid through the catheter. The decay time for a unit fluid pressure pulse generated within the catheter is proportional to the catheter length, i.e. as the catheter length increases so does the decay time. The catheter length can therefore be estimated based on, e.g., the decay time of the pressure pulse. The estimated catheter length can also be analyzed to determine if it is representative of the actual catheter length, or, e.g., is affected by one or more catheter malfunctions including cuts and occlusions. Systems for automatically estimating the catheter length include an implantable catheter, a pumping mechanism, a pressure sensor configured to measure pressure within the catheter, and a processor that calculates the catheter length from a pressure pulse measured while delivering fluid through the catheter.

This application is a continuation of U.S. application Ser. No.12/433,836, filed Apr. 30, 2009, the entire content of which isincorporated herein by reference.

TECHNICAL FIELD

The disclosure relates generally to implantable medical devices and,more particularly, to implantable fluid delivery systems.

BACKGROUND

A variety of medical devices are used for chronic, i.e., long-term,delivery of fluid therapy to patients suffering from a variety ofconditions, such as chronic pain, tremor, Parkinson's disease, epilepsy,urinary or fecal incontinence, sexual dysfunction, obesity, spasticity,or gastroparesis. For example, pumps or other fluid delivery devices canbe used for chronic delivery of therapeutic agents, such as drugs topatients. These devices are intended to provide a patient with atherapeutic output to alleviate or assist with a variety of conditions.Typically, such devices are implanted in a patient and provide atherapeutic output under specified conditions on a recurring basis.

One type of implantable fluid delivery device is a drug infusion devicethat can deliver a fluid medication to a patient at a selected site. Adrug infusion device may be implanted at a location in the body of apatient and deliver a fluid medication through a catheter to a selecteddelivery site in the body. Drug infusion devices, such as implantabledrug pumps, commonly include a reservoir for holding a supply of thetherapeutic substance, such as a drug, for delivery to a site in thepatient. The fluid reservoir can be self-sealing and accessible throughone or more ports. A pump is fluidly coupled to the reservoir fordelivering the therapeutic substance to the patient. A catheter providesa pathway for delivering the therapeutic substance from the pump to thedelivery site in the patient.

Implantable drug delivery or infusion devices are commonly used, forexample, when chronic administration of a pharmaceutically active agentor therapeutic substance to a patient is required. An implantableinfusion pump-catheter delivery system may be preferred when it isimportant to deliver the agent to a specific site or when the agent mustbe administered on a recurrent basis in small, controlled dosages.Precise delivery of appropriate amounts of a fluid agent requireaccurate characterization of various delivery components of the system,such as the reservoir and catheter. The volume of the catheter, which isproportional to its length, is needed by the clinician or programmer,e.g., when determining the amount of drug needed to prime the entirepump and catheter. Catheter volume is also important when determiningthe time duration used to maintain a current infusion rate (bridgingperiod) after refilling the pump's reservoir with a different fluidagent or fluid agent concentration to insure that the new infusion ratedoes not go into effect until the current drug has exited the catheter.Ordinarily, clinicians measure and record the length of catheter duringimplantation which is then used in calculation priming and bridgingdurations. However, there are a variety of ways for which thisinformation fails to be recorded or is some how lost leaving cliniciansthat must manage patient's drug delivery therapies without the catheterlength information vital to insuring that the prime and bridgeprocedures are safe.

SUMMARY

In general, this disclosure describes techniques for automaticallyestimating the length of a catheter of an implanted medical device. Thetechniques may measure a pressure decay response to pumping fluid dosesthrough the catheter. The decay time for a unit fluid pressure pulse isproportional to the length of the catheter. In particular, as the lengthof the catheter increases, so does the pressure decay time. The catheterlength can therefore be estimated based on the decay time of thepressure pulse.

In one example, a method includes delivering an amount of fluid throughan implantable catheter. A pressure within a lumen of the catheter ismeasured during the delivery of the fluid to the patient. An estimatedlength for the catheter is calculated based on the measured pressure.

In another example, an implantable fluid delivery system includes afluid delivery pump, a catheter, a pressure sensor, and a processor. Thecatheter is connected to the fluid delivery pump. The pressure sensor isarranged to measure a pressure in a lumen of the catheter. The processoris configured to control the fluid delivery pump to deliver an amount offluid through the catheter, control the pressure sensor to measure apressure within a lumen of the catheter during the delivery of the fluidthrough the catheter, and calculate an estimated length of the catheterbased on the measured pressure.

In another example, a computer-readable medium contains instructions forcausing a programmable processor to control a fluid delivery pump todeliver an amount of fluid through an implantable catheter, control apressure sensor to measure a pressure within a lumen of the catheterduring the delivery of the fluid through the catheter, and calculate anestimated length of the catheter based on the measured pressure.

In still another example, a device includes means for delivering anamount of fluid through an implantable catheter, means for measuring apressure within a lumen of the catheter during the delivery of the fluidthrough the catheter, and means for calculating an estimated length ofthe catheter based on the measured pressure.

The details of one or more examples disclosed herein are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages will be apparent from the description anddrawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram illustrating an example of a fluiddelivery system including an implantable fluid delivery deviceconfigured to deliver a fluid therapeutic agent to a patient via acatheter.

FIG. 2 is functional block diagram illustrating an example of theimplantable fluid delivery device of FIG. 1.

FIG. 3 is a flowchart illustrating a method of determining a length ofan implantable catheter used with an implantable fluid delivery device.

FIG. 4 is a plot of pressure versus time that illustrates the pressurewithin an implanted catheter during the delivery of a fluid dose to apatient.

FIG. 5 is a flow chart illustrating examples by which a pressure pulsewithin an implantable catheter during the delivery of a fluid dose maybe measured.

FIG. 6 is a plot of catheter length versus pressure pulse decay timethat illustrates the relationship of catheter length to pressure withinthe delivery lumen of a normally functioning catheter.

FIG. 7 is a flowchart illustrating examples for determining if anestimated length calculated according to disclosed examples isrepresentative of the actual length of a catheter.

FIG. 8 is a plot of pressure versus time that illustrates the pressurewithin a cut or leaking catheter during a delivery of a fluid dose to apatient.

FIG. 9 is a partial cross-sectional view of an example capacitivepressure sensor.

FIG. 10 is a partial cross-sectional view of an example inductivepressure sensor.

DETAILED DESCRIPTION

Medical devices are useful for treating, managing or otherwisecontrolling various patient conditions or disorders including, e.g.,pain (e.g., chronic pain, post-operative pain or peripheral andlocalized pain), tremor, movement disorders (e.g., Parkinson's disease),diabetes, epilepsy, neuralgia, chronic migraines, urinary or fecalincontinence, sexual dysfunction, obesity, gastroparesis, mooddisorders, or other disorders. Some medical devices, referred to hereingenerally as fluid delivery devices may be configured to deliver one ormore fluid therapeutic agents, alone or in combination with othertherapies, such as electrical stimulation, to one or more target siteswithin a patient. For example, in some cases, a fluid delivery devicemay deliver pain-relieving drugs) to patients with chronic pain, insulinto a patient with diabetes, or other fluids to patients with differentdisorders. The device may be implanted in the patient for chronictherapy delivery (i.e., longer than a temporary, trial basis) ortemporary delivery.

As used in this disclosure, the terms dosing program or therapy programgenerally refer to a program sent to an implantable fluid deliverydevice by a programming device to cause the fluid delivery device todeliver fluid at a certain rate and at a certain time. The dosingprogram may include, for example, definitions of a priming bolus, abridging bolus, a supplemental bolus, and a therapy schedule. A dosingprogram may include additional information, such as patient information,permissions for a user to add a supplemental bolus, as well as limits onthe frequency or number of such boluses, historical therapy schedules,fluid or drug information, or other information.

The term therapy schedule may generally refer to a rate (which may bezero) at which to administer a fluid, or a drug or drug combinationwithin the fluid, at specific times to a patient. In particular, thetherapy schedule may define one or more programmed doses, which may beperiodic or aperiodic, each dose including, e.g., a rate of fluiddelivery and a time duration for which to deliver the dose. Dosegenerally refers to the amount of drug delivered over a period of time,and may change over the course of a therapy schedule such that a drugmay be delivered at different rates at different times.

A priming bolus refers to a bolus delivered by the implantable fluiddelivery device to move the fluid to the distal tip of the catheter,e.g., the tip of the catheter that is remote from the reservoir andinternal tubing. Once the fluid is primed to the distal tip of thecatheter, the device is ready to deliver fluid to the patient from thedistal tip, e.g., via one or more fluid outlets at or near the distaltip. The device delivers the priming bolus during a priming phase toprepare the device for delivery of the fluid to the patient.

In addition to a priming bolus, an implantable fluid delivery device mayalso perform a bridging bolus, which can also be referred to as abridge. Bridges are performed when a new fluid is inserted into areservoir of the device while an old fluid is still present in thedevice, e.g., within internal tubing of the device and/or within acatheter connected to the device. The bridge is performed to define arate at which to deliver the old fluid until the old fluid is completelydelivered out of the catheter and to the patient such that the devicecontains only the new fluid.

In general, there may be only one priming bolus per implanted device,because only at or around implantation of the device in the patient isthe fluid pathway between the device and the target delivery sitepartially or completely free from any fluid such that priming isnecessary. Boluses after the priming bolus that prepare the device todeliver a therapeutic agent to the patient, i.e. fill the fluid pathwayfrom the device to the target delivery site with the agent, may bereferred to as bridging boluses.

A supplemental bolus is a bolus administered to the patient outside ofthe therapy schedule. The terms independent bolus, one-time bolus, andtherapeutic bolus may also be used in this disclosure to refer to asupplemental bolus. In one example, the implantable fluid deliverydevice may administer a supplemental bolus before the implantable fluiddelivery device begins administering doses of fluid according to thetherapy schedule. In another example, the implantable fluid deliverydevice may administer a supplemental bolus during the therapy schedule,e.g., to override or supplement the therapy schedule in response toclinician instruction or patient request.

One task required with implantable infusion therapy systems iscalculating priming and/or bridging boluses. Miscalculations of primingor bridging bolus amount can result in undesirable overdose or underdosescenarios. A parameter used to calculate the duration of a bridge orpriming bolus is the volume of the catheter, which is directlyproportional to the implanted catheter length. The catheter length mayvary according to the distance between the implantable pump and a targetdelivery site within the patient.

It is typically the responsibility of the implanting physician tomanually measure and record the catheter length. In many cases,implantable catheters come in standard lengths and are cut to fit aparticular implantation. In such cases, the physician may measure thelength cut off from the catheter at implantation and calculate thelength of the implanted catheter by subtracting the cut off length fromthe full length of the catheter prior to being cut. In any event, ifthis recording is done in error or not at all, then the subsequent boluscalculations will also be subject to error, which in turn can lead toconditions including overdosing or underdosing the patient in which thedevice is implanted. In some instances, the catheter and/or fluiddelivery device may need to be explanted or surgically exposed tomeasure the length of the catheter.

It may be difficult to deliver appropriate priming and/or bridgingboluses when the recorded catheter length is not accurate. Techniquesdescribed in this disclosure may incorporate the use of therapy systemswith one or more pressure sensors configured to measure pressuresomewhere within the fluid pathway of an implantable fluid deliverydevice, such as a catheter extending from the device and/or internaltubing within the device. Such techniques may be performed when thecatheter is implanted or outside the body prior to implantation. In somecases, pressure may be measured while a dose of the therapeutic agent isdelivered to the patient.

The fluid delivery device in such therapy systems, and/or another devicesuch as an external programmer, may be configured to automaticallycalculate an estimated length of the catheter based on the measuredpressure. Automatic calculation of the length of the catheter withoutrelying on human intervention, e.g., without requiring the implantingphysician to measure and record length, can promote accurate catheterlength recording. Hence, in some cases, cathether length may beestimated automatically instead of by the clinician. Accurate catheterlength recording may promote proper delivery of priming and bridgingboluses, or other doses in which accurate determination of catheterlength is important to proper and safe delivery of the therapeutic agentto the patient.

FIG. 1 is a conceptual diagram illustrating an example of a therapysystem 10, which includes implantable medical device (IMD) 12, catheter18, and external programmer 20. IMD 12 is connected to catheter 18 todeliver at least one therapeutic agent, such as a pharmaceutical agent,pain relieving agent, anti-inflammatory agent, gene therapy agent, orthe like, to a target site within patient 16. Example therapeutic agentsthat IMD 12 can be configured to deliver include, but are not limitedto, insulin, morphine, hydromorphone, bupivacaine, clonidine, otheranalgesics, genetic agents, antibiotics, nutritional fluids, hormones orhormonal drugs, gene therapy drugs, anticoagulants, cardiovascularmedications or chemotherapeutics.

In the example of FIG. 1, the therapeutic agent is a therapeutic fluid,which IMD 12 delivers to patient 16 through catheter 18 from proximalend 18A coupled to IMD 12 to distal end 18B located proximate to thetarget site. Catheter 18 can comprise a unitary catheter or a pluralityof catheter segments connected together to form an overall catheterlength. External programmer 20 is configured to wirelessly communicatewith IMD 12 as needed, such as to provide or retrieve therapyinformation or control aspects of therapy delivery (e.g., modify thetherapy parameters such as rate or timing of delivery, turn IMD 12 on oroff, and so forth) from IMD 12 to patient 16.

IMD 12, in general, may have an outer housing that is constructed of abiocompatible material that resists corrosion and degradation frombodily fluids including, e.g., titanium or biologically inert polymers.IMD 12 may be implanted within a subcutaneous pocket relatively close tothe therapy delivery site. For example, in the example shown in FIG. 1,IMD 12 is implanted within an abdomen of patient 16. In other examples,IMD 12 may be implanted within other suitable sites within patient 16,which may depend, for example, on the target site within patient 16 forthe delivery of the therapeutic agent. In still other examples, IMD 12may be external to patient 16 with a percutaneous catheter connectedbetween IMD 12 and the target delivery site within patient 16.

Catheter 18 may be coupled to IMD 12 either directly or with the aid ofa catheter extension (not shown in FIG. 1). In the example shown in FIG.1, catheter 18 traverses from the implant site of IMD 12 to one or moretargets proximate to spine 14. Catheter 18 is positioned such that oneor more fluid delivery outlets (not shown in FIG. 1) of catheter 18 areproximate to the one or more targets within patient 16. In the exampleof FIG. 1, IMD 12 delivers a therapeutic agent through catheter 18 toone or more targets proximate to spinal cord 14. IMD 12 can beconfigured for intrathecal drug delivery into the intrathecal space orepidural delivery into the epidural space, both of which surround spinalcord 14. The epidural space (also known as “extradural space” or“peridural space”) is the space within the spinal canal (formed by thesurrounding vertebrae) lying outside the dura mater, which encloses thearachnoid mater, subarachnoid space, the cerebrospinal fluid, and spinalcord 14. The intrathecal space is within the subarachnoid space, whichis past the epidural space and dura mater and through the theca.

Although the target site shown in FIG. 1 is proximate to spinal cord 14of patient 16, other applications of therapy system 10 includealternative target delivery sites. The target delivery site in otherapplications of therapy system 10 can be located within patient 16proximate to, e.g., sacral nerves (e.g., the S2, S3, or S4 sacralnerves) or any other suitable nerve, organ, muscle or muscle group inpatient 16, which may be selected based on, for example, a patientcondition. In one such application, therapy system 10 may be used todeliver a therapeutic agent to tissue proximate to a pudendal nerve, aperineal nerve or other areas of the nervous system, in which cases,catheter 18 would be implanted and substantially fixed proximate to therespective nerve. Positioning catheter 18 to deliver a therapeutic agentto various sites within patient 16 enables therapy system 10 to assistin managing, e.g., peripheral neuropathy or post-operative painmitigation, ilioinguinal nerve therapy, intercostal nerve therapy,gastric stimulation for the treatment of gastric motility disordersand/or obesity, and muscle stimulation, or for mitigation of otherperipheral and localized pain (e.g., leg pain or back pain). As anotherexample delivery site, catheter 18 may be positioned to deliver atherapeutic agent to a deep brain site or within the heart (e.g.,intraventricular delivery of the agent). Delivery of a therapeutic agentwithin the brain may help manage any number of disorders or diseasesincluding, e.g., depression or other mood disorders, dementia,obsessive-compulsive disorder, migraines, obesity, and movementdisorders, such as Parkinson's disease, spasticity, and epilepsy.Catheter 18 may also be positioned to deliver insulin to a patient withdiabetes.

As already mentioned, therapy system 10 can be used to reduce painexperienced by patient 16. In such an application, IMD 12 can deliverone or more therapeutic agents to patient 16 according to one or moredosing programs that set forth different therapy parameters, such as atherapy schedule specifying programmed doses, dose rates for theprogrammed doses, and specific times to deliver the programmed doses.The dosing programs may be a part of a program group for therapy, wherethe group includes a plurality of dosing programs and/or therapyschedules. In some examples, IMD 12 may be configured to deliver atherapeutic agent to patient 16 according to different therapy scheduleson a selective basis. IMD 12 may include a memory to store one or moretherapy programs, instructions defining the extent to which patient 16may adjust therapy parameters, switch between dosing programs, orundertake other therapy adjustments. Patient 16 or a clinician mayselect and/or generate additional dosing programs for use by IMD 12 viaexternal programmer 20 at any time during therapy or as designated bythe clinician.

In some examples, multiple catheters 18 may be coupled to IMD 12 totarget the same or different tissue or nerve sites within patient 16.Thus, although a single catheter 18 is shown in FIG. 1, in otherexamples, system 10 may include multiple catheters or catheter 18 maydefine multiple lumens for delivering different therapeutic agents topatient 16 or for delivering a therapeutic agent to different tissuesites within patient 16. Accordingly, in some examples, IMD 12 mayinclude a plurality of reservoirs for storing more than one type oftherapeutic agent. In some examples, IMD 12 may include a single longtube that contains the therapeutic agent in place of a reservoir.However, for ease of description, an IMD 12 including a single reservoiris primarily discussed herein with reference to the example of FIG. 1.

Programmer 20 is an external computing device that is configured tocommunicate with IMD 12 by wireless telemetry. For example, programmer20 may be a clinician programmer that the clinician uses to communicatewith IMD 12. Alternatively, programmer 20 may be a patient programmerthat allows patient 16 to view and modify therapy parameters. Theclinician programmer may include additional or alternative programmingfeatures than the patient programmer. For example, more complex orsensitive tasks may only be allowed by the clinician programmer toprevent patient 16 from making undesired or unsafe changes to theoperation of IMD 12.

Programmer 20 may be a hand-held computing device that includes adisplay viewable by the user and a user input mechanism that can be usedto provide input to programmer 20. For example, programmer 20 mayinclude a display screen (e.g., a liquid crystal display or a lightemitting diode display) that presents information to the user. Inaddition, programmer 20 may include a keypad, buttons, a peripheralpointing device, touch screen, voice recognition, or another inputmechanism that allows the user to navigate though the user interface ofprogrammer 20 and provide input.

If programmer 20 includes buttons and a keypad, the buttons may bededicated to performing a certain function, i.e., a power button, or thebuttons and the keypad may be soft keys that change in functiondepending upon the section of the user interface currently viewed by theuser. Alternatively, the screen (not shown) of programmer 20 may be atouch screen that allows the user to provide input directly to the userinterface shown on the display. The user may use a stylus or theirfinger to provide input to the display.

In other examples, rather than being a handheld computing device or adedicated computing device, programmer 20 may be a larger workstation ora separate application within another multi-function device. Forexample, the multi-function device may be a cellular phone, personalcomputer, laptop, workstation computer, or personal digital assistantthat can be configured with an application to simulate programmer 20.Alternatively, a notebook computer, tablet computer, or other personalcomputer may enter an application to become programmer 20 with awireless adapter connected to the personal computer for communicatingwith IMD 12.

When programmer 20 is configured for use by the clinician, programmer 20may be used to transmit initial programming information to IMD 12. Thisinitial information may include hardware information for system 10 suchas the type of catheter 18, the position of catheter 18 within patient16, the length of catheter 18 at implantation as measured by theimplanting physician and/or as automatically estimated by IMD 12, thetype of therapeutic agent(s) delivered by IMD 12, a baseline orientationof at least a portion of IMD 12 relative to a reference point, therapyparameters of therapy programs stored within IMD 12 or within programmer20, and any other information the clinician desires to program into IMD12.

The clinician uses programmer 20 to program IMD 12 with one or moretherapy programs that define the therapy delivered by the IMD. During aprogramming session, the clinician may determine one or more dosingprograms that may provide effective therapy to patient 16. Patient 16may provide feedback to the clinician as to efficacy of a program beingevaluated or desired modifications to the program. Once the clinicianhas identified one or more programs that may be beneficial to patient16, the patient may continue the evaluation process and determine whichdosing program or therapy schedule best alleviates the condition of thepatient or otherwise provides efficacious therapy to the patient.

The dosing program information may set forth therapy parameters, such asdifferent predetermined dosages of the therapeutic agent (e.g., a doseamount), the rate of delivery of the therapeutic agent (e.g., rate ofdelivery of the fluid), the maximum acceptable dose, a time intervalbetween successive supplemental boluses such as patient-initiatedboluses (e.g., a lock-out interval), a maximum dose that may bedelivered over a given time interval, and so forth. IMD 12 may include afeature that prevents dosing the therapeutic agent in a mannerinconsistent with the dosing program. Programmer 20 may assist theclinician in the creation/identification of dosing programs by providinga methodical system of identifying potentially beneficial therapyparameters.

A dosage of a therapeutic agent, such as a drug, may be expressed as anamount of drug, e.g., measured in milligrams or other volumetric units,provided to patient 16 over a time interval, e.g., per day ortwenty-four hour period. In this sense, the dosage may indicate a rateof delivery. This dosage amount may convey to the caregiver anindication of the probable efficacy of the drug and the possibility ofside effects. In general, a sufficient amount of the drug should beadministered in order to have a desired therapeutic effect, such as painrelief. However, the amount of the drug administered to the patientshould be limited to a maximum amount, such as a maximum daily dose, inorder not to avoid potential side effects. Program information specifiedby a user via programmer 20 may be used to control dosage amount, dosagerate, dosage time, maximum dose for a given time interval (e.g., daily),or other parameters associated with delivery of a drug or other fluid byIMD 12.

In some cases, programmer 20 may also be configured for use by patient16. When configured as the patient programmer, programmer 20 may havelimited functionality in order to prevent patient 16 from alteringcritical functions or applications that may be detrimental to patient16. In this manner, programmer 20 may only allow patient 16 to adjustcertain therapy parameters or set an available range for a particulartherapy parameter. In some cases, a patient programmer may permit thepatient to control IMD 12 to deliver a supplemental, patient bolus, ifpermitted by the applicable therapy program administered by the IMD,e.g., if delivery of a patient bolus would not violate a lockoutinterval or maximum dosage limit. Programmer 20 may also provide anindication to patient 16 when therapy is being delivered or when IMD 12needs to be refilled or when the power source within programmer 20 orIMD 12 need to be replaced or recharged.

Whether programmer 20 is configured for clinician or patient use,programmer 20 may communicate to IMD 12 or any other computing devicevia wireless communication. Programmer 20, for example, may communicatevia wireless communication with IMD 12 using radio frequency (RF)telemetry techniques. Programmer 20 may also communicate with anotherprogrammer or computing device via a wired or wireless connection usingany of a variety of communication techniques including, e.g., RFcommunication according to the 802.11 or Bluetooth specification sets,infrared (IR) communication according to the IRDA specification set, orother standard or proprietary telemetry protocols. Programmer 20 mayalso communicate with another programming or computing device viaexchange of removable media, such as magnetic or optical disks, ormemory cards or sticks including, e.g., non-volatile memory. Further,programmer 20 may communicate with IMD 12 and another programmer via,e.g., a local area network (LAN), wide area network (WAN), publicswitched telephone network (PSTN), or cellular telephone network, or anyother terrestrial or satellite network appropriate for use withprogrammer 20 and IMD 12.

In accordance with catheter length determination techniques described indetail below with reference to FIGS. 3-8, therapy system 10 of FIG. 1may include one or more pressure sensors that are configured to measurea pressure pulse within catheter 18 during the delivery of a fluid dose.The fluid dose may be an actual fluid dose delivered within patient 16,a test dose delivered outside the patient, or a test dose of a testfluid delivered within the patient. IMD 12 includes control electronics,e.g. one or more processors and non-volatile memories that areconfigured to receive the measured pressure pulse within catheter 18from the pressure sensor(s) and calculate an estimated length ofcatheter 18 based on the pressure pulse.

Measuring the pressure pulse within catheter 18 can include determininga maximum pressure within the catheter and determining a decay time ofthe pressure pulse. The decay time of a pressure pulse, generallyspeaking, is the time required for the pressure pulse within thecatheter to fall from a first pressure level to a second pressure level.The first pressure level may be a maximum pressure level sensed upondelivery of a fluid amount and the second pressure level may be abaseline pressure. The baseline pressure level may be a pressure sensedbefore of after the delivery of the fluid amount. IMD 12 can beconfigured to measure a baseline pressure within the catheter while nofluid dose is being delivered to the patient. In some examples, thebaseline pressure can be measured prior to delivering the fluid dose tothe patient through the catheter.

Calculating the estimated length of the catheter can include, e.g.,calculating the estimated length of the catheter based on the decaytime. The estimated length may be calculated automatically, e.g., by IMD12 or programmer 20, avoiding the need for human intervention. In somecases, the estimated length may be calculated automatically and manuallyby a human caregiver, such that the automated calculation may be used asa cross-verification check to ensure that the human-measured length isgenerally accurate. Pressure measurement to estimate catheter length maybe performed when the catheter is implanted in the patient (in vivo) orwhen the catheter is outside the patient prior to implantation (exvivo).

Additionally, IMD 12 can be configured to analyze the estimated lengthto determine if it is representative of an actual length of thecatheter. In some cases, catheter 18 may become disconnected from IMD 12or otherwise malfunction due to, e.g., cuts or occlusions in thecatheter. IMD 12 can therefore discern whether the estimated length thatis calculated based on the pressure pulse is representative of theactual length of catheter 18 by identifying conditions indicative of oneor more catheter malfunctions. For example, IMD 12 can determine if themaximum pressure of the pressure pulse is below a minimum pressurethreshold value, which may indicate the presence of an air bubble in thefluid pathway or that catheter 18 is disconnected completely from IMD12. Additionally, IMD 12 can determine if the decay time is below aminimum threshold value, which may indicate a leak in catheter 18. Inanother example, IMD 12 can determine if the decay time is above amaximum threshold value, which may indicate an occlusion in catheter 18.IMD 12 can also analyze the pressure pulse by determining if thepressure within the lumen falls below a baseline pressure after decayingfrom a maximum pressure, which may indicate either a cut in catheter 18or that catheter 18 is disconnected from IMD 12.

In general, the estimated length may be used to generate a catheterlength value for presentation to a user, such as a clinician, e.g., viaa user interface external programmer 20. The clinician then may selectthe estimated catheter length for use by programmer 20 and/or IMD 12 incalculating appropriate dosage parameters, including parameters forpriming and/or bridging boluses. In some cases, the estimated catheterlength produced based on the pressure measurements may be used as across-verification check to ensure that a previously determined catheterlength, such as a catheter length determined by a clinician or otheruser, is reasonably accurate.

If the automatically estimated catheter length and the catheter lengthestimated by the clinician are inconsistent, programmer 20 and/or IMD 12may be configured to use the automatically estimated catheter length tocompute or recompute dosage parameters, or the programmer and/or IMD maypresent an alert to the clinician or other user indicating theinconsistency. In some cases, although the catheter length determinationmay be described as automatic, a user such as a clinician may controlprogrammer 20 and/or IMD 12 to initiate the automated catheter lengthdetermination at a desired time.

FIG. 2 is a functional block diagram illustrating components of anexample of IMD 12, which includes processor 26, memory 28, telemetrymodule 30, fluid delivery pump 32, reservoir 34, refill port 36,internal tubing 38, pressure sensor 40, catheter access port 42, andpower source 44. Processor 26 is communicatively connected to memory 28,telemetry module 30, fluid delivery pump 32, and pressure sensor 40.Fluid delivery pump 32 is connected to reservoir 34 and internal tubing38. Reservoir 34 is connected to refill port 36. Pressure sensor 40 isarranged in internal tubing 38 between pump 32 and catheter access port42. Measurements obtained by pressure sensor 40 may be received byprocessor 26 and stored in memory 28 by the processor. Catheter accessport 42 is connected to internal tubing 38 and catheter 18. IMD 12 alsoincludes power source 44, which is configured to deliver operating powerto various components of the IMD.

During operation of IMD 12, processor 26 controls fluid delivery pump 32with the aid of instructions associated with program information that isstored in memory 28 to deliver a therapeutic agent to patient 16 viacatheter 18. Instructions executed by processor 26 may, for example,define dosing programs that specify the amount of a therapeutic agentthat is delivered to a target tissue site within patient 16 fromreservoir 30 via catheter 18. The instructions may further specify thetime at which the agent will be delivered and the time interval overwhich the agent will be delivered. The amount of the agent and the timeover which the agent will be delivered are a function of, oralternatively determine, the dosage rate at which the fluid isdelivered. The therapy programs may also include other therapyparameters, such as the frequency of bolus delivery, the type oftherapeutic agent delivered if IMD 12 is configured to deliver more thanone type of therapeutic agent, and so forth. Components described asprocessors within IMD 12, external programmer 20, or any other devicedescribed in this disclosure may each comprise one or more processors,such as one or more microprocessors, digital signal processors (DSPs),application specific integrated circuits (ASICs), field programmablegate arrays (FPGAs), programmable logic circuitry, or the like, eitheralone or in any suitable combination.

Upon instruction from processor 26, fluid delivery pump 32 draws fluidfrom reservoir 34 and pumps the fluid through internal tubing 38 tocatheter 18 through which the fluid is delivered to patient 16 to effectone or more of the treatments described above. Internal tubing 38 is asegment of tubing or a series of cavities within IMD 12 that run fromreservoir 34, around or through fluid delivery pump 32 to catheteraccess port 42. Fluid delivery pump 32 can be any mechanism thatdelivers a therapeutic agent in some metered or other desired flowdosage to the therapy site within patient 16 from reservoir 30 viaimplanted catheter 18.

In one example, fluid delivery pump 32 can be a squeeze pump thatsqueezes internal tubing 38 in a controlled manner, e.g., such as aperistaltic pump, to progressively move fluid from reservoir 34 to thedistal end of catheter 18 and then into patient 16 according toparameters specified by a set of program information stored on memory 28and executed by processor 26. Fluid delivery pump 32 can also be anaxial pump, a centrifugal pump, a pusher plate, a piston-driven pump, orother means for moving fluid through internal tubing 38 and catheter 18.In one particular example, fluid delivery pump 32 can be anelectromechanical pump that delivers fluid by the application ofpressure generated by a piston that moves in the presence of a varyingmagnetic field and that is configured to draw fluid from reservoir 34and pump the fluid through internal tubing 38 and catheter 18 to patient16.

Periodically, fluid may need to be supplied transcutaneously toreservoir 34 because all of a therapeutic agent has been or will bedelivered to patient 16, or because a clinician wishes to replace anexisting agent with a different agent or similar agent with differentconcentrations of therapeutic ingredients. Refill port 26 can thereforecomprise a self-sealing membrane to prevent loss of therapeutic agentdelivered to reservoir 30 via refill port 26. For example, after apercutaneous delivery system, e.g., a hypodermic needle, penetrates themembrane of refill port 26, the membrane may seal shut when the needleis removed from refill port 26.

Pump 32 may deliver fluid in controlled pulses. When pump 32 is calledupon by processor 26 to deliver a fluid dose through catheter 18,processor 26 may also control pressure sensor 40 to measure a pressurepulse within catheter 18 that is generated by the delivery of fluidthrough catheter 18. Pressure sensor 40 can also measure a steady statebaseline pressure within catheter 18 when no fluid dose is beingdelivered to patient 16. As will be discussed in greater detail below,pressure sensor 40 can be any of a number of types of sensors that arecapable of measuring the pressure within an implantable catheterincluding, e.g. capacitive and inductive pressure sensors.

Pressure sensor 40 can read either gauge or absolute pressure of thefluid in catheter 18. Because methods disclosed herein rely oncomparison of pressure just prior to and during delivery of a fluid doseto a patient through the catheter 18, changes in ambient pressure may beof reduced importance in implementing examples according to thisdisclosure (especially where the successive pulses are delivered withinrelatively short time frames, e.g., within minutes or even seconds ofeach other).

In those instances where, however, it is desirable to use pressuremeasurements from sensor 40 that are adjusted to account for ambientpressure outside of the catheter 18, a reference pressure may bedetected within the body of patient 16 in which catheter 18 is implantedor may be detected outside of the patient's body. When detected withinthe body of patient 16, a reference pressure may be detected in alocation near IMD 12 or the target delivery site near distal end 18B ofcatheter 18, or even in a location in a separate area of the patient'sbody. A reference pressure may be obtained in any location capable ofproviding a pressure indicative of the external environment of implantedcatheter 18.

In one example, an infusion system may include a catheter 18 having afirst lumen for delivering a fluid and a second lumen through which nofluid is delivered. A reference pressure may then be detected in thesecond lumen. The second lumen in catheter 18 can be used to obtain areference pressure from distal end 18B of catheter 18, from a regionnear the target delivery site of therapy system 10. An expandedexplanation of reference pressures may be found in commonly assignedU.S. Pat. No. 7,320,676, entitled PRESSURE SENSING IN IMPLANTABLEMEDICAL DEVICES, by Miesel.

The pressure pulse(s) measured by pressure sensor 40 during delivery ofa fluid dose to patient 16 can be stored in memory 28. Processor 26 canaccess one or more stored pressure pulses to calculate an estimatedlength of catheter 18 based on an algorithm, group of algorithms, or aseries of executable steps in any form, which may also be stored inmemory 28. Processor 26 may also rely on one or more look-up tables, orother data aggregations stored in memory 28 to calculate the estimatedlength of catheter 18. For example, processor 26 may retrieve storedpressure pulses from a look-up table in memory 28. Processor 26 may alsoanalyze the estimated length calculation to determine whether theestimate is representative of an actual length of catheter 18. Analyzingthe estimated length can be executed using the same or a differentalgorithm used for calculating the estimated length, which may also bestored in memory 28 of IMD 12. Memory 28 may include any volatile ornon-volatile media, such as a random access memory (RAM), read onlymemory (ROM), non-volatile RAM (NVRAM), electrically erasableprogrammable ROM (EEPROM), flash memory, and the like.

Although controlling fluid pump 32 and sensor 40 to deliver fluidthrough catheter 18 and measure a pressure pulse therein has beendescribed as executed by IMD 12, and in particular, processor 26, inother examples one or more of these functions may be carried out byother devices including, e.g., external programmer 20. For example, auser may interact with external programmer 20 to command processor 26 ofIMD 12 to control fluid pump 32 to deliver a fluid dose to patient 16through catheter 18. Thereafter, pressure sensor 40 may measure apressure pulse within catheter 18 during delivery of the fluid dose andprocessor 26 may control telemetry module 30 to transmit the measuredpressure pulse data to external programmer 20. External programmer 20may, in some cases, store the measured pressure pulse data in one ormore non-volatile memories included in the programmer. In any event, aprocessor housed within programmer 20 may calculate an estimated lengthof catheter 18 based on the measured pressure pulse received from IMD 12in accordance with the techniques described below with reference toFIGS. 3-8.

In addition to storing pressure pulse measurement data obtained bypressure sensor 40 and one or more algorithms used to execute examplemethods according to this disclosure, memory 28 may store programinformation including instructions for execution by processor 26, suchas, but not limited to, therapy programs, historical therapy programs,timing programs for delivery of fluid from reservoir 34 to catheter 18,and any other information regarding therapy of patient 16. A program mayindicate the bolus size or flow rate of the drug, and processor 26 mayaccordingly deliver therapy. A program may also indicate the frequencyat which pressure sensor 40 is commanded to measure a pressure pulseand, as will be discussed in greater detail below, instructions forestimating the length of catheter 18 based on one or more measuredpressure pulses. Memory 28 may include separate memories for storinginstructions, patient information, therapy parameters (e.g., groupedinto sets referred to as “dosing programs”), therapy adjustmentinformation, program histories, and other categories of information suchas any other data that may benefit from separate physical memorymodules. Therapy adjustment information may include information relatingto timing, frequency, rates and amounts of patient boluses or otherpermitted patient modifications to therapy. In some examples, memory 28stores program instructions that, when executed by processor 26, causeIMD 12 and processor 26 to perform the functions athibuted to them inthis disclosure.

At various times during the operation of IMD 12 to treat patient 16,communication to and from IMD 12 may be necessary to, e.g., changetherapy programs, adjust parameters within one or more programs,configure or adjust a particular bolus, send or receive an estimatedlength of catheter 18, or to otherwise download information to or fromIMD 12. Processor 26 therefore controls telemetry module 30 towirelessly communicate between IMD 12 and other devices including, e.g.programmer 20. Telemetry module 30 in IMD 12, as well as telemetrymodules in other devices described herein, such as programmer 20, can beconfigured to use RF communication techniques to wirelessly send andreceive information to and from other devices respectively. In addition,telemetry module 30 may communicate with programmer 20 via proximalinductive interaction between IMD 12 and the external programmer.Telemetry module 30 may send information to external programmer 20 on acontinuous basis, at periodic intervals, or upon request from theprogrammer.

Power source 44 delivers operating power to various components of IMD12. Power source 44 may include a small rechargeable or non-rechargeablebattery and a power generation circuit to produce the operating power.In the case of a rechargeable battery, recharging may be accomplishedthrough proximal inductive interaction between an external charger andan inductive charging coil within IMD 12. In some examples, powerrequirements may be small enough to allow IMD 12 to utilize patientmotion and implement a kinetic energy-scavenging device to tricklecharge a rechargeable battery. In other examples, traditional batteriesmay be used for a limited period of time. As another alternative, anexternal inductive power supply could transcutaneously power IMD 12 asneeded or desired.

FIG. 3 is a flow chart illustrating an example method of determining alength of an implantable catheter. Method 50 includes measuring abaseline pressure within a lumen of the catheter while no fluid dose isbeing delivered to the patient (step 52), delivering a fluid dose to apatient through the catheter (step 54), measuring a pressure pulsewithin the lumen of the catheter during the delivery of the fluid doseto the patient (step 56), calculating an estimated length of thecatheter based on the measured pressure pulse (step 58), and analyzingthe estimated length to determine if it is representative of an actuallength of the catheter (step 60). Thereafter, or at any other timeduring the execution of methods according to this disclosure measuredpressure pulse, estimated length, and data relating to the analysis ofthe estimated length may be stored temporarily or permanently stored in,e.g., a non-volatile memory.

Method 50, as well as other examples according to this disclosure can beimplemented using, e.g. processor 26 of IMD 12 shown in FIGS. 1 and 2 toexecute one or more algorithms stored in memory 28. In other examples,however, method 50 may be carried out using the control electronics,e.g. internal memory and processor of an external fluid delivery devicethat controls delivery of a therapeutic agent through a transcutaneouscatheter. In still another example, method 50 may be carried outcooperatively by more than one device including, e.g., externalprogrammer 20 and IMD 12, which receives pressure measurement data fromIMD 12 by wireless telemetry. Method 50 is generally directed toautomatically determining, i.e. without the need for human interaction,the length of an implantable catheter based on the characteristics of apressure pulse generated within the catheter during delivery of a fluiddose to a patient. Determining the length of the implanted catheterwithout relying on the implanting physician measuring and recordinglength can assist in and improve proper delivery of, e.g., priming andbridging boluses, or any other doses in which catheter length iscritical to proper and safe delivery of the therapeutic agent to thepatient.

One task required with implantable infusion therapies is the calculationof priming and/or bridging boluses. Miscalculation of the bolus durationcan result in undesirable overdose or underdose scenarios. A keyparameter used to calculate the duration of a bridge or priming bolus isthe volume of the catheter, which is directly proportional to theimplanted catheter length. In particular, given a known catheter lumendiameter, the catheter length is used to calculate catheter volume. Itis typically the responsibility of the implanting physician to manuallymeasure and record the catheter length. However, if this recording isdone in error or not at all, then the subsequent bolus calculations willalso be subject to error, which in turn can lead to conditions includingoverdosing or underdosing the patient in which the device is implanted.

Examples disclosed herein therefore may augment or replace the need formanually entering the catheter length in implantable fluid deliverysystems, reducing the possibility of miscalculating dosing durationsincluding, e.g. the duration of a bridge or priming bolus. These examplemethods and systems can be important when a patient changes clinics oris out of the area in which his or her clinic is located and needs tohave a pump refilled with a new drug, in which case the patient may needa bridging bolus. If the catheter type and length is not available, thismethod may allow a reasonable estimate of the dead space to becalculated for the switch over from one drug to another.

Referring again to FIG. 3, method 50 initially involves processor 26controlling pressure sensor 40 to measure a baseline pressure within thelumen of catheter 18 while no fluid dose is being delivered to patient16 (step 52). In order to establish a reference pressure within catheter18, method 50 may include measuring the pressure within the lumen of thecatheter while it is full of fluid but not during delivery of any fluiddose to the patient. Measuring the pressure under these conditions willestablish the baseline pressure from which other conditions can bejudged, e.g., the pressure pulse generated within the catheter duringdelivery of a fluid dose to the patient. The baseline pressure can bemeasured (step 52) by pressure sensor 40 any time before or afterprocessor 26 controls fluid delivery pump 32 to deliver a fluid dose toa patient through catheter 18 (step 54). For example, the baselinepressure can be measured upon implantation of therapy system 10 after apriming bolus has been delivered to fill catheter 18 with fluid. Inanother example, the baseline pressure can be measured just prior tofluid pump 32 delivering a fluid dose to patient 16 (step 54). In stillanother example, the baseline pressure can be measured before and afterfluid pump 32 delivers the fluid dose (step 54) and processor 26controls pressure sensor 40 to measure the pressure pulse (step 56).

In addition to measuring the baseline pressure (step 52), method 50includes processor 26 controlling fluid delivery pump 32 to deliver afluid dose to patient 16 through catheter 18 (step 54). As discussedwith reference to therapy system 10 in FIGS. 1 and 2, catheter 18 may beconnected to IMD 12, which includes fluid pump 32 configured to delivera fluid therapeutic agent to patient 16 through the catheter. Inexamples according to this disclosure, a single stroke of fluid pump 32may be configured to deliver a nominal fluid dose or pulse to patient 16through catheter 18 with a relatively small volume and little or notherapeutic affect on the patient. For example, the volume of fluidagent delivered to patient 16 from a single stroke of one example pumpmechanism could be as small as 1 micro liter. In this way, for purposesof determining the length of catheter 18, the fluid dose can bedelivered to patient 16 through the catheter at virtually any time withlittle to no affect on the patient's therapy program or schedule.

The timing and frequency of the fluid dose that is delivered to thepatient for purposes of examples according to this disclosure can varydepending on the intended application. For example, a nominal fluid dosecould be delivered to the patient for purposes of initially determiningthe length of the catheter once shortly after the catheter is implantedwithin the patient, e.g., before activation of therapy programs. In anyevent, timing and frequency can be controlled by the control electronicswithin the IMD to which the implantable catheter is connected, e.g.processor 26 and memory 28 of IMD 12. For example, IMD 12 used inaccordance with examples disclosed herein can include memory 28 thatstores one or more algorithms that include instructions executed byprocessor 26 for periodically delivering nominal fluid doses to patient16 in order to measure a pressure pulse within catheter 18 that is thenused to estimate the length of catheter 18.

In another example, the algorithms can query when and how often thetherapy program for IMD 12 delivers therapeutic doses to patient 16 todetermine if additional nominal doses are necessary for purposes ofestimating the length of catheter 18. In other words, IMD 12 can beconfigured to integrate the functions associated with examples ofdetermining the length of catheter 18 with the execution of the therapyprogram of patient 16 by measuring the pressure within catheter 18during delivery of a regularly scheduled therapeutic dose to thepatient. The calculated length can then be used, i.e., plugged into,equations used to compute dosages for the therapy programs. Inparticular, each program may take into account catheter volume, as afunction of catheter length, in computing dosages to be delivered topatient 16 in the ordinary course of therapy.

Method 50 also includes measuring a pressure pulse within the lumenduring the delivery of the fluid dose (step 56). As described withreference to IMD 12 in FIGS. 1 and 2, pressure sensor 40 fluidlyconnected to implanted catheter 18 can measure the pressure within thelumen of catheter 18 at any time that fluid pump 32 is called upon todeliver a fluid dose to patient 16. Pressure sensor 40 can be any of anumber of types of sensors that are capable of measuring the pressurewithin an implantable catheter including. Examples of pressure sensorsinclude capacitive and inductive pressure sensors. The pressure measuredby sensor 40 can be gauge or absolute pressure, but, in any event, willinclude a characteristic transient pressure pulse with a maximumpressure that is generated when the fluid dose is delivered as thepressure in catheter 18 increases slightly in reaction to the dosebefore settling back to a steady state, i.e., baseline pressure.

In order to better understand the response of an implantable catheter tofluid pressure inputs, a canine study was performed that, inter alta,measured pressure pulses within an implanted intrathecal catheter withvarying lengths and/or defects. FIG. 4 illustrates a pressure pulserepresentative of results of this study. FIG. 4 is a plot of pressureversus time that illustrates the pressure within an implanted catheterduring the delivery of a fluid dose to a patient. In FIG. 4, transientpressure pulse 64 is generated when the fluid dose is delivered throughcatheter 18 to patient 16. Pressure pulse 64 includes a maximumpressure, P_(max) to which the pressure within catheter 18 climbs almostinstantaneously from the baseline pressure, P_(B) after the pumpmechanism 32 of IMD 12 begins delivering the fluid dose to patient 16.Pressure pulse 64 also includes a total duration, t_(T), and a decaytime, t_(D). The decay time t_(D) is the time required for the pressurepulse within the lumen of catheter 18 to fall from the maximum pressureP_(max) to the baseline pressure P_(B). Because the pressure rises fromthe baseline pressure P_(B) to the maximum pressure P_(max) almostinstantaneously, in many cases the total duration t_(T) of pressurepulse 64 may be almost equal to the decay time t_(D).

As illustrated in FIG. 4, the pressure pulse within the lumen that ismeasured in methods according to this disclosure has several distinctcharacteristics. Therefore, in the examples disclosed herein, measuringthe pressure pulse (step 56) can include, e.g., processor 26 of IMD 12further analyzing the pressure measurement data received from pressuresensor 40 to determine one or more of the maximum pressure (step 70),the decay time (step 72), and the total duration (step 74) of thepressure pulse as illustrated in the flow chart of FIG. 5.

It was discovered from the canine study that the pressure pulsewaveform, such as the pressure pulse 64 shown in FIG. 4, for a normallyfunctioning catheter with a length of approximately 80 centimetersgenerated a maximum pressure of about 10.66 KPa. The catheter used inthe study was model number 8709 manufactured by Medtronic, Inc. ofMinneapolis, Minn. The rise from the baseline pressure was nearlyinstantaneous relative to a 250 hertz sampling rate that was used tomeasure pressure within the catheter. The form of the decay from themaximum pressure back to the baseline was exponential (as illustrated inFIG. 4) with a decay constant on the order of 0.038 seconds and totaldecay time approximately equal to 0.100 seconds.

Referring again to FIG. 3, in addition to measuring a pressure pulsewithin the lumen during the delivery of the fluid dose (step 56), method50 includes calculating an estimated length of catheter 18, e.g., viaprocessor 26, based on the measured pressure pulse (step 58). Thepressure change within the lumen of catheter 18 caused by delivering afluid dose therethrough, i.e., the pressure pulse, is a function of, atleast in part, the length of the catheter. Therefore, the length of aproperly functioning implanted catheter 18 can be estimated withreasonable accuracy based on the magnitude and/or characteristics of thepressure pulse measured within the lumen during the delivery of thefluid dose to patient 16 (step 56).

For example, calculating the estimated length of catheter 18 based onthe measured pressure pulse (step 58) can include processor 26calculating the estimated length based on the decay time (t_(D)) of thepressure pulse. As explained above with reference to FIG. 4, the decaytime of the pressure pulse can, in many cases, be virtually equal to thetotal duration of the pressure pulse (t_(T)) because the pressure risesalmost instantaneously from the baseline (P_(B)) to a maximum pressure(P_(max)). FIG. 6 is a plot of catheter length versus pressure pulsedecay time that illustrates the relationship of length to pressurewithin the delivery lumen of a normally functioning catheter. The curveshown in FIG. 6 is representative of experimental data generated bymeasuring pressure pulse decay times for a catheter as the length of thecatheter was varied. In general, the length of the implanted catheter isdirectly proportional to the decay time of the pressure pulse generatedduring delivery of a fluid dose through the catheter. As illustrated inFIG. 6, the length of the catheter increases linearly as the decay timeof the pressure pulse increases, and decreases linearly as the decaytime decreases.

In the canine study discussed above with reference to FIG. 4, animplanted intrathecal catheter was continuously cut to shorten thecatheter from full length to near-zero length (e.g. cut at or near theexit port from the IMD to which the catheter was connected). As thelocation of the cut in the catheter moved closer to the exit port of theIMD, i.e. as the catheter length decreased, the decay time of thepressure pulse decreased from 0.100 seconds down to zero seconds in alinear manner. As a percentage of the full length of the implantedcatheter, the decay time of the pressure pulse decreased from 0.100seconds at full length to 0.060 seconds at 80% of full length, 0.040seconds at 50% of full length, and 0.020 seconds at 25% of full length.Using an experimental relationship such as that illustrated in FIG. 6and described with reference to the canine study, the estimated lengthof catheter 18 can be calculated by processor 26 based on the decay timeof the pressure pulse generated within the lumen of catheter 18 whenfluid pump 32 delivers the fluid dose to patient 16.

In general, calculating the estimated length of catheter 18 may dependnot only on properties or characteristics of the measured pressurepulse, but also on the particular fluidic resistance characteristics ofthe catheter including, e.g., fluidic resistance per unit length andvolumetric compliance per unit length. In some examples, catheter 18 maybe of a known type with known characteristics that may be stored, e.g.,in memory 28 of IMD 12 (or in another location, e.g. memory on externalprogrammer 20). In such examples, processor 26 of IMD 12 may retrievekey characteristics (e.g. fluidic resistance per unit length andvolumetric compliance per unit length) of catheter 18 from a look-uptable stored in memory 28 or memory in programmer 20. Processor 26 maythen account for the catheter characteristics retrieved from memory 28in the calculation of the estimated length of catheter 18. In otherexamples, catheter 18 may be of a known type, but with unknowncharacteristics. The type of catheter 18 may, in such examples, bestored in memory 28 of IMD 12. In another example, neither the type norany of the characteristics of catheter 18 are known and/or stored. Inexamples where either or both of the type or characteristics of catheter18 are unknown and/or unrecorded, catheter characteristics relevant tolength calculations may be assumed to provide a range boundary to theestimated length calculation. In any of the examples incorporatingcatheter characteristics, IMD 12 may communicate with externalprogrammer 20 to provide an alert or other notification to a user, e.g.patient and/or clinician, that prompts the user to, e.g., provide oraugment any missing or incorrect catheter information or notifies theuser whenever any assumptions are applied to the estimated lengthcalculations.

Using the fluidic resistance characteristics of catheter 18 and theabove described characteristics of the measured pressure pulse, thelength of catheter 18 may be estimated analytically instead of byreferencing experimental data as described above with reference to FIG.6. In such cases, the overall pump catheter system including, e.g. fluidpump 32 and catheter 18 may be approximated by an electrical analog thatrepresents fluidic restriction (e.g. represented as resistance) andcompliance (e.g. represented as capacitance), and pressure (e.g.represented as power) of the various components of and at variouslocations in the system model. In some examples, the dominantcharacteristics of the electrical analog model are the fluidicresistance and compliance of catheter 18. The input signal to the systemmay be a relatively rapid, e.g. less than approximately 0.005 seconds,pressure pulse that is assumed to be a unit-impulse response to a fluiddose delivered through catheter 18. Within the overall system model,catheter 18 may itself behave as a distributed restriction andcompliance model having an equivalent model that may be represented by asingle or lumped restriction and compliance value including, e.g., thetotal fluidic resistance of the catheter.

The length of catheter 18 can be estimated by determining the totalfluidic resistance of the catheter, because the total fluidic resistanceis equal to the length of the catheter multiplied by a fluidicresistance per unit length constant for the catheter. In general, thefluidic resistance per unit length constant of catheter 18 is dependenton the size, shape, and material properties of the catheter lumen. Inone example system model, the instantaneous flow rate of fluid withincatheter 18 is equal to the differential pressure across the length ofthe catheter divided by the total fluidic resistance of the catheter.The volume of fluid dispensed into catheter 18 is equal to the integralof flow rate over time and may be assumed to be a constant including,e.g. a nominal dose equal to approximately 1 micro liter. Therefore, thedispensed volume is equal to a constant, which is equal to the integralof the differential pressure across the length of catheter 18 divided bythe total fluidic resistance of the catheter over time. Compliance inthe overall system and in catheter 18 is the physical property thatcreates the exponential decay of the measured pressure pulse illustratedand described above with reference to FIG. 4. The integral of themeasured exponential pressure pulse over time is proportional to thevolume of fluid dispensed through catheter 18. In particular, theintegral of the measured pressure pulse is equal to the total fluidicresistance of catheter 18 multiplied by the dispensed volume. Therefore,the total fluidic resistance of catheter 18 is equal to the integral ofthe measured pressure pulse divided by the volume of dispensed fluid,i.e. is equal to a constant including, e.g. 1 micro liter. The totalfluidic resistance of catheter 18 can therefore be calculated from themeasured pressure pulse and the known volume of the fluid dose deliveredto patient 12. The length of catheter 18 is equal to total fluidicresistance divided by the fluidic resistance per unit length constantfor the catheter.

Method 50 also may include analyzing the estimated length to determineif it is representative of an actual length of catheter 18 (step 60).For example, processor 26 of IMD 12 can analyze the estimated lengthcalculation to determine if it is representative of the actual length ofcatheter 18, or if it has been affected by, e.g., one or more defects inor malfunctions of catheter 18. FIG. 7 is a flowchart illustrating anumber of examples by which processor 26 of IMD 12 may determine if theestimated length calculated in examples disclosed herein isrepresentative of the actual length of catheter 18 (step 60). In oneexample shown in FIG. 7, processor 26 can compare the decay time (t_(D))of the pressure pulse generated within the lumen of catheter 18 to aminimum threshold value to determine if the catheter is disconnectedfrom IMD 12 or contains a cut or leak proximate IMD 12 (step 80). In asimilar manner, processor 26 can also compare the estimated length(L_(est)) to a minimum threshold value to determine if catheter 18 isdisconnected or contains a cut or leak (step 82). If catheter 18 is cutproximate to or is disconnected from IMD 12, the estimated lengthcalculated by processor 26 based on the measured pressure pulse will berelatively small because the decay time of the pressure pulse, which isproportional to the length of catheter 18, will have a smaller magnitudethan a normally functioning catheter. Therefore, processor 26 of IMD 12can compare the calculated estimated length of catheter 18 and/or thedecay time of the pressure pulse to a minimum value to determine ifcatheter 18 is disconnected or has a leak and therefore is notexhibiting a length estimate that is representative of the actual lengthof catheter 18.

The magnitude of the minimum length threshold to which the estimatedlength of catheter 18 is compared may vary depending on, e.g. the typeof therapy being delivered and the implantation location of IMD 12 andthe target drug delivery site within patient 16. For example, an averagecatheter length for intrathecal fluid drug delivery may be approximatelyequal to 80 centimeters. The average or typical length for catheter 18may be used to set a threshold minimum that represents catheter lengthsthat are outside of range above and below the typical length.

In another example, processor 26 of IMD 12 can compare a maximumpressure within the lumen of catheter 18 to a minimum threshold value todetermine if catheter 18 is disconnected from IMD 12 or contains an airbubble (step 84). As explained with reference to FIG. 4, during thedelivery of a fluid dose to patient 16 a characteristic pressure pulsewill exhibit a maximum pressure, P_(max), to which the pressure withincatheter 18 climbs from a baseline pressure, P_(B), after the fluid pump32 of IMD 12 begins delivering the fluid dose to patient 16. In the casecatheter 18 is disconnected from IMD 12 or contains an air bubble, themaximum pressure of the pressure pulse may have an unusually low value.

Generally speaking, the decay time of the measured pressure pulse isdirectly proportional to the maximum pressure, making the magnitude ofthe pressure generated within catheter 18 during the delivery of a fluiddose directly proportional to the length. Therefore, lower maximumpressures will generally indicate shorter decay times and shorterlengths of catheter 18. A minimum pressure threshold may be set thatrepresents pressures indicating lengths of catheter 18 that areunusually low for the particular therapeutic application of IMD 12. Aswith the minimum length threshold described above, the average ortypical length for catheter 18 may be used to set a threshold minimumpressure that represents pressure values that will yield catheterlengths that are outside of range above and below the typical length.

In addition to referencing average or typical catheter lengths, themagnitude of the minimum pressure threshold may be set based onexperimental data for different types of catheters of varying lengths.For example, in the above referenced canine study it was determined thatthe maximum pressure for the a cut or disconnected catheter havingnear-zero length falls off from around 10.66 KPa for a full lengthnormally functioning catheter to a value around 4 KPa. Additionally, thestudy showed that air bubbles trapped within the pressure circuit, e.g.within catheter 18 dampen the magnitude of the maximum pressure of thepressure pulse by a factor of 10. Once a minimum pressure thresholdvalue is set, processor 26 of IMD 12 can compare the maximum pressure ofthe pressure pulse measured by pressure sensor 40 during delivery of thefluid dose to patient 16 to the minimum to determine if catheter 18 maybe disconnected from IMD 12 or contains an air bubble and therefore isnot exhibiting a length estimate that is representative of the actuallength of catheter 18.

In still other examples, processor 26 of IMD 12 can compare the decaytime of the pressure pulse generated within the lumen of catheter 18 toa maximum threshold value to determine if catheter 18 includes anocclusion (step 86). In some examples in which catheter 18 includes anocclusion along the length thereof, the pressure pulse generated withincatheter 18 during delivery of a fluid dose to patient 16 may reach amaximum pressure, but may not decay to the baseline pressure before asubsequent fluid dose delivery. In such examples, a second dosedelivered to patient 16 by fluid pump 32 may then raise the pressure inthe lumen of catheter 18 to a second maximum followed by a drop toanother elevated pressure that is again above the baseline pressure.This transient pressure pulse behavior may be caused by the fluid beingtrapped within catheter 18 by the blockage. Because the pressure withincatheter 18 does not drop to the baseline pressure, the decay timecannot be measured based on the pressure data provided by pressuresensor 40. In such examples, processor 26 of IMD 12 can compare thedecay time of the pressure pulse measured by pressure sensor 40 duringdelivery of the fluid dose to patient 16 to a maximum to determine ifcatheter 18 is occluded and therefore is not exhibiting a lengthestimate that is representative of the actual length of catheter 18.

Still referring to FIG. 7, in another example, processor 26 of IMD 12can compare a minimum pressure within catheter 18 during delivery of afluid dose to patient 16 to the baseline pressure to determine ifcatheter 18 has a cut or leak (step 88). In some cases where catheter 18includes a cut or leak very close to IMD 12, the pressure pulsegenerated during delivery of the fluid dose may exhibit a characteristicnegative minimum pressure below the baseline pressure as illustrated inFIG. 8. FIG. 8 is a plot of pressure versus time that illustrates thepressure within an implanted catheter during the delivery of a fluiddose to a patient. The pressure pulse shown in FIG. 8 is representativeof a catheter that has a cut close to a proximal end thereof connectedto an IMD. In FIG. 8, transient pressure pulse 100 is generated when thefluid dose is delivered through catheter 18 to patient 16. Pressurepulse 100 includes a maximum pressure to which the pressure withincatheter 18 climbs from the baseline pressure, P_(B), after the pumpmechanism 32 of IMD 12 begins delivering the fluid dose to patient 16.

In contrast to the example that compares the maximum pressure to aminimum threshold described above with reference to step 84 of FIG. 7,the maximum pressure of pressure pulse 100 reaches a level typicallyassociated with a properly functioning catheter 18. However, pressurepulse 100 decays to a minimum pressure, P_(min), that is less than thebaseline pressure. The negative minimum pressure illustrated by pressurepulse 100 in FIG. 8 may occur because a catheter cut close to or at theproximal connection to the IMD behaves like an undamped system in whichthe pressure drops more rapidly from the maximum pressure and thereforeovershoots the baseline pressure by a non-trivial amount. For example,in the canine study previously referenced, the pressure pulse for a cutcatheter rose and decayed quickly, and then dropped below the baselinepressure by approximately 1.33 KPa. In examples including a pressurepulse with such a characteristic negative minimum pressure, processor 26of IMD 12 can compare a minimum pressure of the pressure pulse measuredby pressure sensor 40 to a baseline pressure to determine if catheter 18is cut and therefore is not exhibiting a length estimate that isrepresentative of the actual length of catheter 18.

In the event that processor 26 of IMD 12 determines that the analysisperformed in all of steps 80-88 illustrated in FIG. 7 produces negativeresults, processor 26 may conclude that catheter 18 contains nomalfunctions (step 90) and that the estimated length is representativeof the actual length of the catheter. If positive results are produced,processor 26 may detect a catheter malfunction and generate anappropriate alert to the patient and/or clinician. The alert may be atactile, audible or visible alert presented via the IMD 12 or externalprogrammer 20.

Once the estimated length of catheter 18 has been analyzed and deemedsatisfactory by, e.g., the process illustrated in FIG. 7, IMD 12 maystore the estimated length in memory 28 and/or present the length to auser. For example, an implanting physician may use therapy system 10including catheter length estimation means according to this disclosureto confirm the proper implantation of catheter 18 within patient 16. Insuch examples, processor 26 of IMD 12 may implement a method accordingto this disclosure to calculate an estimated length of catheter 18 afterthe physician has implanted therapy system 10 within patient 16. IMD 12may store the estimated length of catheter 18 in memory 28 from which itmay be accessed and/or transmitted to other devices by processor 26.

For example, processor 26 may transmit the estimated length calculationto external programmer 20 to be displayed via a user interface thereon.In this way, the implanting physician, or any other authorized user mayaccess the estimated length calculated by IMD 12. In some cases, theestimated length calculation may be used by processor 26 to compute orrecompute dosages for therapy programs. In other cases, if the estimatedlength calculation is generally consistent with a previously estimatedlength calculation, e.g., by the clinician or otherwise, processor 26takes no action and does not compute or recompute dosages. Hence,processor 26 may generate an alert in the event a malfunction isdetected, or generate an alert in the event the estimated catheterlength is inconsistent with a previous catheter length determination.

FIGS. 9 and 10 show examples of pressure sensor appropriate for use withmethods and systems according to this disclosure. FIG. 9 is a partialcross-sectional view of an example pressure sensor 40A including upperdiaphragm 110, lower diaphragm 112, sensor casing 114, conductivematerial 116, complementary conductive material 118, stationaryinsulator 120, coupler 122 and capacitive sensor 124. In FIG. 9, sensor40A is a capacitive flow sensor utilizing two diaphragms 110 and 112.Upper diaphragm 110 is mounted to sensor casing 114. Upper diaphragm 110may be made from or coated, at least partially, with a conductivematerial 116. Complementary conductive material 118 may be coated onstationary insulator 120, which may, in some examples, be a sapphireinsulator. In one example, a 0.002 inch gap is created betweenconductive materials 116 and 118. Using air as an insulator, conductivematerials 116 and 118 form a capacitor. As upper diaphragm 110 moves inresponse to pressure changes, the capacitance created by conductivematerials 116 and 118 also changes.

A similar arrangement exists on the opposite end of sensor casing 114with lower diaphragm 112. Conductive materials 116 and 118 are coated onlower diaphragm 112 and sapphire insulator 120, respectively, forminganother capacitor. Coupler 122 is positioned for relative movement with,preferably against, both upper diaphragm 110 and lower diaphragm 112.Capacitive sensors 124 are sensitive to changes in capacitances frommovement of each of upper and lower diaphragms 112 and may provide thepressure measurements from within catheter 18 used in examples accordingto this disclosure.

The dual diaphragm and dual capacitor arrangement described withreference to FIG. 9 actually multiplies the amount of change incapacitance with a given amount of movement in diaphragms 110 and 112.Since the pressure changes are small, the movement of diaphragms 110 and112 are small. The capacitance change is additive resulting in twice theperformance. In one example, coupler 126 contacts diaphragms 110 and 112in the center of the diaphragms in order to obtain the maximum movementof the diaphragms. Coupler 126 should not significantly inhibit themovement of diaphragms 110 and 112.

FIG. 10 illustrates example pressure sensor 40B which operates on achange in inductance and includes upper diaphragm 110, lower diaphragm112, coupler 122, center primary coil 130, upper secondary coil 132,lower secondary coil 134, and magnetic element 136. In FIG. 10, sensor40B again has two diaphragms 110 and 112 with coupler 122 mounted formovement therebetween. A center primary coil 130 is arranged isconfigured to be excited with an alternating current. Upper and lowersecondary coils, 132 and 134 respectively, are mounted above and belowprimary coil 130, respectively. Magnetic element 136 is mounted formovement with coupler 122. As magnetic element 136 moves up and/or downin response to changes in pressure, the inductance induced in secondarycoils 132 and 134 varies. An inductance sensor (not shown) can detectthe change in these inductances and provide an output indicative of achange in pressure. Again, this arrangement doubles the effectiveness ofmovement in diaphragms 110 and 112 by additively combining the changesin inductance of each individual secondary coil 132, 134.

Examples presented in this disclosure may augment or replace the needfor manually entering the catheter length in implantable fluid deliverysystems, thereby promoting accuracy in the calculation of dosingdurations including, e.g. the duration of a bridge or priming bolus. Thetechniques described herein include therapy systems with one or morepressure sensors configured to measure pressure somewhere within an IMDfluid pathway (e.g. a catheter attached thereto, and/or internal tubeswithin the IMD) while a dose of the therapeutic agent is delivered tothe patient. The IMD in such therapy systems, and/or another device maybe configured to calculate an estimated length of the catheter based onthe measured pressure.

Calculating the length of the catheter without relying on humanintervention (e.g. implanting physician measuring and recording length)can assist in and improve properly delivering, e.g., priming andbridging boluses, or any other doses in which catheter length iscritical to proper and safe delivery of the therapeutic agent to thepatient. In some cases, the automatically determined catheter length maybe used in computing doses, or used as a cross-verification check toensure the reasonable accuracy of catheter length determinations made bya clinician or other caregiver. The disclosed methods and systems can beimportant when a patient changes clinics or is out of the area in whichhis or her clinic is located and needs to have a pump refilled with anew drug, i.e. needs a bridging bolus. If the catheter type and lengthis not available, this method would allow a reasonable estimate of thedead space to be calculated for the switch over from one drug toanother.

The techniques described in this disclosure may be implemented, at leastin part, in hardware, software, firmware or any combination thereof. Forexample, various aspects of the described techniques may be implementedwithin one or more processors, including one or more microprocessors,digital signal processors (DSPs), application specific integratedcircuits (ASICs), field programmable gate arrays (FPGAs), or any otherequivalent integrated or discrete logic circuitry, as well as anycombinations of such components. The term “processor” or “processingcircuitry” may generally refer to any of the foregoing logic circuitry,alone or in combination with other logic circuitry, or any otherequivalent circuitry. A control unit comprising hardware may alsoperform one or more of the techniques of this disclosure.

Such hardware, software, and firmware may be implemented within the samedevice or within separate devices to support the various operations andfunctions described in this disclosure. In addition, any of thedescribed units, modules or components may be implemented together orseparately as discrete but interoperable logic devices. Depiction ofdifferent features as modules or units is intended to highlightdifferent functional aspects and does not necessarily imply that suchmodules or units must be realized by separate hardware or softwarecomponents. Rather, functionality associated with one or more modules orunits may be performed by separate hardware or software components, orintegrated within common or separate hardware or software components.

The techniques described in this disclosure may also be embodied orencoded in a computer-readable medium, such as a computer-readablestorage medium, containing instructions. Instructions embedded orencoded in a computer-readable medium may cause a programmableprocessor, or other processor, to perform the method, e.g., when theinstructions are executed. Computer readable storage media may includerandom access memory (RAM), read only memory (ROM), programmable readonly memory (PROM), erasable programmable read only memory (EPROM),electronically erasable programmable read only memory (EEPROM), flashmemory, a hard disk, a CD-ROM, a floppy disk, a cassette, magneticmedia, optical media, or other computer readable media.

Various examples have been described herein. These and other examplesare within the scope of the following claims.

1. An implantable fluid delivery system comprising: a fluid deliverypump; a catheter connected to the fluid delivery pump; a pressure sensorarranged to measure a pressure within a lumen of the catheter; and aprocessor that: controls the fluid delivery pump to deliver an amount offluid through the catheter; controls the pressure sensor to measure apressure within the lumen of the catheter during the delivery of thefluid through the catheter; calculates a first length of the catheterfrom a connection to the fluid delivery pump to a distal end of thecatheter based on the measured pressure; receives a second length of thecatheter that is provided by a user; compares the first length and thesecond length; and sets one or more therapy parameters according towhich the processor controls the fluid delivery pump to deliver thefluid, based on the comparison.
 2. The system of claim 1, wherein theprocessor sets the one or more therapy parameters at least by settingone or more dosage parameters according to which the processor controlsthe fluid delivery pump to deliver the fluid, based on the comparison.3. The system of claim 2, wherein the processor compares the firstlength and the second length at least by determining whether the firstlength is approximately equal to the second length, and wherein, in theevent the comparison indicates that the first length is notapproximately equal to the second length, the processor sets the one ormore dosage parameters based on the first length.
 4. The system of claim3, wherein the processor sets the one or more dosage parameters at leastby computing a bolus volume based on the first length.
 5. The system ofclaim 3, wherein, in the event the comparison indicates that the firstlength is approximately equal to the second length, the processorcontrols the fluid delivery pump to deliver the fluid based on one ormore dosage parameters previously set based on the second length.
 6. Thesystem of claim 3, wherein, in the event the comparison indicates thatthe first length and the second length are not approximately equal, theprocessor generates an alarm.
 7. The system of claim 1, wherein tocalculate the first length of the catheter, the processor: determines amaximum pressure within the lumen; determines a decay time required forthe pressure within the lumen to fall from the maximum pressure to abaseline pressure; and calculates the first length based on the decaytime, wherein the first length and the decay time are directlyproportional.
 8. The system of claim 1, further comprising animplantable medical device that includes the fluid delivery pump and theprocessor.
 9. The system of claim 8, wherein the implantable medicaldevice further includes a memory device, and wherein the processorreceives the second length of the catheter from one of the memory deviceand an external programmer.
 10. The system of claim 1, furthercomprising an external programmer that includes the processor.
 11. Thesystem of claim 10, wherein the external programmer further includes amemory device, and wherein the processor receives the second length ofthe catheter from one of the memory device and a user input.
 12. Acomputer-readable medium comprising instructions for causing aprogrammable processor in an implantable fluid delivery system to:control a fluid delivery pump to deliver an amount of fluid through animplantable catheter connected to the fluid delivery pump; control apressure sensor to measure a pressure within a lumen of the catheterduring the delivery of the fluid through the catheter; calculate a firstlength of the catheter from a connection to the fluid delivery pump to adistal end of the catheter based on the measured pressure; receive asecond length of the catheter that is provided by a user; compare thefirst length and the second length; and set one or more therapyparameters according to which the processor controls the fluid deliverypump to deliver the fluid, based on the comparison.
 13. Thecomputer-readable medium of claim 12, wherein the instructions thatcause the processor to set the one or more therapy parameters compriseinstructions that cause the processor to at least set one or more dosageparameters according to which the processor controls the fluid deliverypump to deliver the fluid, based on the comparison.
 14. Thecomputer-readable medium of claim 13, wherein the instructions thatcause the processor to compare the first length and the second lengthcomprise instructions that cause the processor to at least determinewhether the first length is approximately equal to the second length,and wherein, the instructions that cause the processor to set the one ormore dosage parameters comprise instructions that cause the processorto, in the event the comparison indicates that the first length is notapproximately equal to the second length, set the one or more dosageparameters based on the first length.
 15. The computer-readable mediumof claim 14, wherein the instructions that cause the processor to setthe one or more dosage parameters based on the first length compriseinstructions that cause the processor to at least compute a bolus volumebased on the first length.
 16. The computer-readable medium of claim 14,wherein the instructions that cause the processor to set the one or moredosage parameters further comprise instructions that cause the processorto, in the event the comparison indicates that the first length isapproximately equal to the second length, control the fluid deliverypump to deliver the fluid based on one or more dosage parameterspreviously set based on the second length.
 17. The computer-readablemedium of claim 14, further comprising instructions that cause theprocessor to, in the event the comparison indicates that the firstlength and the second length are not approximately equal, generate analarm.
 18. The computer-readable medium of claim 12, wherein theinstructions that cause the processor to calculate the first length ofthe catheter comprise instructions that cause the processor to:determine a maximum pressure within the lumen; determine a decay timerequired for the pressure within the lumen to fall from the maximumpressure to a baseline pressure; and calculate the first length based onthe decay time, wherein the first length and the decay time are directlyproportional.
 19. The computer-readable medium of claim 12, wherein thecomputer-readable medium is included within one or more of animplantable medical device that includes the fluid delivery pump and theprocessor, and an external programmer that includes the processor.
 20. Adevice comprising: means for delivering an amount of fluid through animplantable catheter; means for measuring a pressure within a lumen ofthe catheter during the delivery of the fluid through the catheter;means for calculating a first length of the catheter from a first end ofthe catheter connected to the means for delivering to a second end ofthe catheter based on the measured pressure; means for receiving asecond length of the catheter that is provided by a user; means forcomparing the first length and the second length; and means for settingone or more therapy parameters according to which the fluid is deliveredthrough the catheter, based on the comparison.